ESM is the term given to the electronic procedures involved with detecting and identifying a threat and other electromagnetic signals as part of a military platform's defensive capabilities. ESM operates passively, and detects radar signals over a broad frequency range, typically over the band 0.5 to 18 GHz. ESM combines several processes in order to operate effectively. These processes include detection, direction finding, signal analysis and parameter estimation. Much of the circuitry currently employed for carrying out these functions is based on analogue technology. A design approach based on multiple narrow-band circuits, which ideally is required to handle the modern radar signal environments, is expensive and therefore it is desirable to minimise the amount of such circuitry in any radar ESM system. The limitations of wide-band receiver systems in handling more than one signal at a time are well known, as is the high cost of analogue channelised architectures.
Digital receiver technology, on the other hand, has a wide range of commercial and other military applications. This potentially offers a much better platform upon which to build future Electronic Support Measures.
The existing digital circuitry cannot cope with the wide frequency coverage requirements of Electronic Support Measures and this has militated against the digital approach, necessitating extensive signal conditioning to map the overall input frequency range into multiple narrower bandwidth channels for digital processing. To handle the full frequency range, it is currently necessary to use multiple digital receivers, which adds significantly to the cost and complexity of any system. The recognised solutions to the problem are to limit the input frequency range, or else adopt a scanning strategy to cover a wider band, but at the cost of extended intercept times.
However, in most channelised architectures for radar ESM, the receiver occupancy at any time is fairly sparse, indicating that the resources available are not well utilised.
Indeed the Radar Electronic Support Measure environment is characterised by sparse occupancy of the signal space in both time and frequency. At any one time, a fairly small number of signals, each of modest bandwidth is incident upon the system
One of the chief challenges in achieving the goal of developing a digital Electronic Support Measure for radar is to process the wide frequency band required with a much narrower bandwidth receiver, of the order of 1-2 GHz. This bandwidth is well within the scope of current digital receiver technology. Current applications of digital receivers are restricted to processing a small part of the total frequency spectrum which is enforced by narrow band pass filters that maintain the fidelity of the digitisation process.
Due to the small number of signals appearing simultaneously, it is possible to exploit the aliasing that occurs in an under sampled (sub-Nyquist) digital receiver to collapse a broad input spectrum into a much smaller spectrum, represented by the processing bandwidth of a digital receiver. However, this aliasing process introduces ambiguities in the frequency measuring process which can severely complicate subsequent analysis of the signal unless they are resolved. At multiples of the sampling frequency, the measured output frequency range folds back on itself, and as noted, several different input frequencies then give rise to the same output frequency (whence the ambiguity).
One way of solving this problem involves splitting the input signal into two signals, subjecting one to a time delay, and running the two signals produced through two channels with the same processors. Phase comparison between the spectra from the two channels will provide an indication of frequency that may resolve the ambiguities. This goes some way towards solving the problem, but still leaves discontinuities at multiples of the sampling frequency, where the frequency of the input signal cannot be determined.